Phase change memory cell and phase change memory

ABSTRACT

A phase change memory cell includes a carbon nanotube layer, a phase change layer, a first electrode, a second electrode, and a third electrode. At least part of the phase change layer is overlapped with the carbon nanotube layer. The first electrode and the second electrode are electrically connected with the carbon nanotube layer, wherein the first electrode and the second electrode are configured to apply a first voltage to the carbon nanotube layer. The third electrode is electrically connected with the phase change layer, wherein the third electrode and the first electrode are configured to apply a second voltage to the phase change layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/332,480, filed on Dec. 21, 2011, entitled, “PHASE CHANGE MEMORYCELL”, which claims all benefits accruing under 35 U.S.C. §119 fromChina Patent Application No. 201110172963.6, filed on Jun. 24, 2011, inthe China Intellectual Property Office, the contents of which are herebyincorporated by reference.

BACKGROUND

1. Technical Field

The present disclosure relates to a phase change memory cell and a phasechange memory.

2. Description of Related Art

Generally, semiconductor memory devices are categorized as random accessmemories (RAMs) or read-only memories (ROMs). ROMs are nonvolatilememory devices such as PROMs (programmable ROMs), EPROMs (erasablePROMs), EEPROMs (electrical EPROMs), and flash memory devices, whichretain their stored data even when their power supplies are interrupted.New types of RAMs containing nonvolatile memory devices have recentlybeen introduced. Examples includes ferroelectric RAMs (FRAMs) employingferroelectric capacitors, magnetic RAMs (MRAMs) employing tunnelingmagneto-resistive (TMR) films, and phase change memories (PCM) usingchalcogenide alloys. Among these, the phase change memory cannot only bewidely used in civilian areas of microelectronics such as mobile phones,digital cameras, MP3 players, and mobile memory, but also has importantapplications in aerospace, missile systems, and military field in thefuture. The phase change memory devices are relatively easy tofabricate, and thus phase change memory devices may provide the bestopportunities in the actual implementation of high-capacity, low costnonvolatile RAMs.

In traditional phase change memory cell, a heating element is used toheat the phase change material. During the dynamic storage, the heatingelement heats the phase change material to produce phase transition.However, one problem is that the heating element is made of metal orsemiconductor, so it is prone to damage or oxidation during the cycle ofheating process. Thus the lifespan of phase change memory cell will beaffected. Furthermore, the heating element is formed by sputtering orvapor deposition, this process is complex and the production cost isrelatively high.

What is needed, therefore, is to provide a phase change memory cell anda phase change memory that can overcome the above-describedshortcomings.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the embodiments can be better understood with referenceto the following drawings. The components in the drawings are notnecessarily drawn to scale, the emphasis instead being placed uponclearly illustrating the principles of the embodiments. Moreover, in thedrawings, like reference numerals designate corresponding partsthroughout the several views.

FIG. 1 shows a schematic top view of one embodiment of a phase changememory cell.

FIG. 2 shows a Scanning Electron Microscope (SEM) image of carbonnanotube film.

FIG. 3 is a flowchart of one embodiment of a method for making a phasechange memory cell.

FIG. 4 shows a schematic top view of one embodiment of a phase changememory cell.

FIG. 5 shows a cross-sectional view along a line of V-V of FIG. 4.

FIG. 6 shows a flowchart of one embodiment of a method for making aphase change memory cell of FIG. 4.

FIG. 7 shows a schematic top view of one embodiment of a phase changememory cell.

FIG. 8 shows a cross-sectional view along a line of VIII-VIII of FIG. 7.

FIG. 9 shows a schematic view of one embodiment of a phase change memorycell including two layers of phase change layer.

FIG. 10 shows a cross-sectional view along a line of X-X of FIG. 9.

FIG. 11 shows a schematic top view of one embodiment of a phase changememory.

FIG. 12 shows a flow chart of one embodiment of a method for making aphase change memory of FIG. 11.

DETAILED DESCRIPTION

The disclosure is illustrated by way of example and not by way oflimitation in the figures of the accompanying drawings in which likereferences indicate similar elements. It should be noted that referencesto “an” or “one” embodiment in this disclosure are not necessarily tothe same embodiment, and such references mean at least one.

Referring to FIG. 1, a phase change memory cell 10 includes a substrate100, a carbon nanotube layer 110, a phase change layer 120, a firstelectrode 132, a second electrode 134, a third electrode 136 and afourth electrode 138. The substrate 100 is used to support the carbonnanotube layer 110, the phase change layer 120, the first electrode 132,the second electrode 134, the third electrode 136 and the fourthelectrode 138. The carbon nanotube layer 110 and the phase change layer120 are stacked together.

The phase change memory cell 10 includes a write circuit and a readcircuit (not shown). The write circuit includes the first electrode 132,the carbon nanotube layer 110, and the second electrode 134 electricallyconnected in series. The write circuit is used to write data into thephase change memory cell 10. The read circuit includes the thirdelectrode 136, the phase change layer 120, and the fourth electrode 138electrically connected in series. The read circuit is used to read datafrom the phase change memory cell 10. Both the write circuit and theread circuit can be used to reset the phase change memory cell 10. Atleast part of the carbon nanotube layer 110 and the phase change layer120 overlap each other.

The substrate 100 is an insulating substrate, and the material of thesubstrate 100 can be porcelain, glass, resin, quartz, or any combinationthereof. The substrate 100 has a character of high temperatureendurance, thus the shape of the substrate 100 can be retained duringthe working process of the phase change memory 10. The size and thethickness of the substrate 100 can be selected according to the workingtemperature of the phase change memory cell 10. In one embodiment, thesubstrate 100 is flexible, and the material of the substrate 100 can beselected form polyimide, phenolic resin, polyester resin, and polyamideresin. In one embodiment, the material of the substrate 100 is polyimidewith a thickness of about 20 μm. The shape of the substrate 100 can beretained under a temperature of 400° C.

The size and shape of the carbon nanotube layer 110 can be selectedaccording to need, for example, the size and the shape can be selectedaccording to the size and shape of the phase change memory cell 10respectively. The shape of the carbon nanotube layer 110 can betriangular, square, rectangular, round, oval or other geometric shapes.In one embodiment, the shape of the carbon nanotube layer 110 isrectangular. The length of the rectangle is in a range from about 50 nmto about 900 μm, and the width of the rectangle is in a range from about20 nm to about 600 μm. In one embodiment, the length is 70 μm, and thewidth is 50 μm. The thickness of the carbon nanotube layer 110 is in arange from about 0.5 nanometer (nm) to about 100 micrometers (μm), suchas 5 nm, 20 nm, 5 μm, 10 μm, 20 μm.

The carbon nanotube layer 110 includes a plurality of carbon nanotubes.The extending direction of the carbon nanotubes is parallel with thesurface of the carbon nanotube layer 110. When the carbon nanotube layer110 is located on the surface of the substrate 100, the extendingdirection of the carbon nanotubes is parallel with the surface of thesubstrate. The carbon nanotube layer 110 can be a continuous film or awire. The carbon nanotubes can be orderly or disorderly aligned. If thecarbon nanotubes are disorderly aligned, the carbon nanotubes can beentangled with each other. If the carbon nanotubes are orderly aligned,the carbon nanotubes can be oriented along one or more preferredorientations. The preferred orientation means that a large majority ofthe carbon nanotubes in the carbon nanotube layer 110 are arrangedsubstantially along the same direction.

Referring to FIG. 2, in one embodiment, the carbon nanotube layer 110includes at least one drawn carbon nanotube film. A drawn carbonnanotube film can be drawn from a carbon nanotube array that is able tohave a film drawn therefrom. The drawn carbon nanotube film includes aplurality of successive and oriented carbon nanotubes joined end-to-endby van der Waals attractive force therebetween. The drawn carbonnanotube film is a free-standing film. Each drawn carbon nanotube filmincludes a plurality of successively oriented carbon nanotube segmentsjoined end-to-end by van der Waals attractive force therebetween. Eachcarbon nanotube segment includes a plurality of carbon nanotubesparallel to each other, and combined by van der Waals attractive forcetherebetween. Some variations can occur in the drawn carbon nanotubefilm. The carbon nanotubes in the drawn carbon nanotube film areoriented along a preferred orientation. The drawn carbon nanotube filmcan be treated with an organic solvent to increase the mechanicalstrength and toughness and reduce the coefficient of friction of thedrawn carbon nanotube film. A thickness of the drawn carbon nanotubefilm can range from about 0.5 nm to about 100 μm. Examples of a carbonnanotube film are taught by U.S. Pat. No. 7,045,108 to Jiang et al., andWO 2007015710 to Zhang et al.

The carbon nanotube layer 110 can include at least two stacked drawncarbon nanotube films. In other embodiments, the carbon nanotube layer110 can include two or more coplanar carbon nanotube films, and caninclude layers of coplanar carbon nanotube films. Additionally, when thecarbon nanotubes in the carbon nanotube film are aligned along onepreferred orientation (e.g., the drawn carbon nanotube film), an anglecan exist between the orientation of carbon nanotubes in adjacent films,whether stacked or adjacent. Adjacent carbon nanotube films can becombined by only the van der Waals attractive force therebetween. Anangle between the aligned directions of the carbon nanotubes in twoadjacent carbon nanotube films can range from about 0 degrees to about90 degrees. When the angle between the aligned directions of the carbonnanotubes is 0 degrees, the two carbon nanotube films can be defined asoriented with the same preferred direction. When the angle between thealigned direction of the carbon nanotubes is greater than 0 degrees andsmaller than 90 degrees or equal to 90 degrees, the two carbon nanotubefilms can be defined as entangled with each other. Stacking the carbonnanotube films will also add to the structural integrity of the carbonnanotube layer 110.

In another embodiment, the carbon nanotube layer 110 can include atleast a pressed carbon nanotube film. The pressed carbon nanotube filmcan be a free-standing carbon nanotube film. The carbon nanotubes in thepressed carbon nanotube film are arranged along a same direction orarranged along different directions. The carbon nanotubes in the pressedcarbon nanotube film can rest upon each other. Adjacent carbon nanotubesare attracted to each other and combined by van der Waals attractiveforce. An angle between a primary alignment direction of the carbonnanotubes and a surface of the pressed carbon nanotube film is 0 degreesto approximately 15 degrees. The greater the pressure applied, thesmaller the angle formed. When the carbon nanotubes in the pressedcarbon nanotube film are arranged along different directions, the carbonnanotubes in the pressed carbon nanotube film have the same properties,such as conductivity, intensity, etc., along the direction parallel tothe surface of the pressed carbon nanotube film/

In another embodiment, the carbon nanotube layer 110 includes aflocculated carbon nanotube film. The flocculated carbon nanotube filmcan include a plurality of long, curved, disordered carbon nanotubesentangled with each other. The carbon nanotubes can be substantiallyuniformly dispersed in the carbon nanotube film. Adjacent carbonnanotubes are acted upon by van der Waals attractive force to form anentangled structure with micropores defined therein. It is understoodthat the flocculated carbon nanotube film is very porous. The length ofthe carbon nanotubes can be greater than 10 μm. Furthermore, due to thecarbon nanotubes in the carbon nanotube layer 110 being entangled witheach other, the carbon nanotube layer 110 employing the flocculatedcarbon nanotube film has excellent durability, and can be fashioned intodesired shapes with a low risk to the integrity of the carbon nanotubelayer 110. The flocculated carbon nanotube film, in some embodiments, isfree standing due to the carbon nanotubes being entangled and adheredtogether by van der Waals attractive force therebetween.

The phase change layer 120 can be partly overlapped with the carbonnanotube layer 110. The surface of the phase change layer 120 isparallel with the surface of the carbon nanotube layer 110.Additionally, the surface of the phase change layer 120 is parallel withthe extending direction of the carbon nanotubes. In one embodiment, thephase change layer 120 is stacked on the carbon nanotube layer 110.Furthermore, a conductive layer (not shown) can be located between thephase change layer 120 and the carbon nanotube layer 110 to transfer theheat produced by the carbon nanotube layer 110 to the phase change layer120. The material of the conductive layer can be selected according toneed such as the Au, Ag, Cu or other conductive materials. The phasechange layer 120 and the carbon nanotube layer 110 can also be spaced ina certain interval. The distance of the interval can be arbitrary andthe heat produced by the carbon nanotube layer 110 can transfer thephase of the phase change layer 120.

The thickness of the phase change layer 120 ranges from about 10 nm toabout 200 nm. The shape of the phase change layer 120 can be triangular,square, rectangular, round, oval or other geometric shapes. The phasechange layer 120 is located within the coverage area of the carbonnanotube layer 110. It means that the area of the phase change layer 120is smaller than that of the carbon nanotube layer 110, and the entirephase change layer 120 is located within the surface of carbon nanotubelayer 110. In one embodiment, the shape of the phase change layer 120 isround with a diameter in a range from about 20 nm to about 250 μm.

The material of the phase change layer 120 can begermanium-antimony-tellurium (Ge—Sb—Te, also referred as “GST”),germanium-tellurium, silicon-antimony-tellurium, silicon-tellurium, orchalcogenide alloys. The phase change layer 120 functions as a variableresistor (i.e., resistance being variable with heat). The phase changematerial is conditioned in one of two stable states, i.e., a crystallinestate or an amorphous state. The phase change material can change phasebetween the crystalline state and the amorphous state, based on the heatproduced by the carbon nanotube layer 110. In one embodiment, thematerial of the phase change layer 120 is GST, and the original state ofthe material is amorphous state with a relatively high resistance. Theamorphous state will be transferred into crystalline state with arelatively low resistance over a crystallization temperature (Tc) in arange from about 200° C. to about 300° C., and the crystalline statewill be transferred into amorphous state in a melting temperature (Tm)in a range from about 400° C. to about 500° C. The melting temperatureis also the reset temperature of the phase change materials.

The terms “crystalline” and “amorphous” are relative terms in thecontext of phase change materials. That is, when a phase change memorycell 10 is said to be in its crystalline state, one skilled in the artwill understand that the phase change material of the cell has a morewell-ordered crystalline structure when compared to its amorphous state.A phase change memory cell 10 in its crystalline state need not be fullycrystalline, and a phase change memory cell 10 in its amorphous stateneed not be fully amorphous. The resistance of the phase changematerials in its crystalline state is smaller than that in its amorphousstate.

The material of the first electrode 132 and the second electrode 134 canbe made of conductive material such as conductive pastes, metal or ITO.The thickness of the first electrode 132 and the second electrode 134can range from about 10 nm to about 100 μm respectively. In oneembodiment, the thickness of the first electrode 132 and the secondelectrode 134 ranges from about 20 nm to about 50 nm respectively. Thematerial of the first electrode 132 and the second electrode 134 is madeof conductive pastes, which includes powdered metal, powdered glass witha low fusion point, and binder. The powdered metal is powdered silver.The binder is terpineol or ethyl cellulose. A weight percentage of thepowdered metal is in a range from about 50% to about 90%. A weightpercentage of the powdered glass with the low fusion point is in a rangefrom about 2% to about 10%. A weight percentage of the binder is in arange from about 8% to about 40%. The first electrode 132 and the secondelectrode 134 are made by printing the conductive pastes onto partsurface of the carbon nanotube layer 110.

The shape, size and the position of the first electrode 132 and thesecond electrode 134 are arbitrary. While applying a voltage between thefirst electrode 132 and the second electrode 134, the carbon nanotubelayer 110 can produce heat to heat the phase change layer 120 to atemperature higher than the crystallization temperature (Tc) or meltingtemperature of the phase change temperature (Tm). In one embodiment, thefirst electrode 132 and the second electrode 134 are located on twoopposite side of the carbon nanotube layer 110. The first electrode 132is electrically connected with a first side of the carbon nanotube layer110. The second electrode 134 is electrically connected with a secondside of the carbon nanotube layer 110. Thus the first electrode 132 andthe second electrode 134 are electrically connected with the entirecarbon nanotube layer 110 respectively. Furthermore, the first electrode132 can be in contact with each carbon nanotube on the first side of thecarbon nanotube layer 110, the second electrode 134 can be in contactwith each carbon nanotube on the second side of the carbon nanotubelayer. Thus the current flowing in the carbon nanotube layer 110 isuniformly dispersed, and the heat efficiency will be improved.

The third electrode 136 and the fourth electrode 138 are electricallyconnected with the phase change layer 120. In one embodiment, the thirdelectrode 136 and the fourth electrode 138 partly cover the surface ofthe phase change layer 120. The phase change layer 120 can be locatedbetween the carbon nanotube layer 110 and both of the third electrode136 and the fourth electrode 138. By applying a voltage on the thirdelectrode 136 and the fourth electrode 138, a current will be introducedinto the phase change layer 120. In one embodiment, the fourth electrode138 can be omitted. It means that a voltage can be either appliedbetween the third electrode 136 and the first electrode 132, or thethird electrode 136 and the second electrode 134 in order to introduce acurrent into the phase change layer 120. Thus the third electrode 136,the phase change layer 120 and the first electrode 132 (or the secondelectrode 134) are electrically connected in series.

The shape and size of the third electrode 136 and the fourth electrode138 are arbitrary and can be selected according to need. The material ofthe third electrode 136 and the fourth electrode 138 are the same as thefirst electrode 132. In one embodiment, the material of the thirdelectrode 136 and the fourth electrode 138 are conductive pastes.

The working process of the phase change memory cell 10 includes threestages: writing data, reading data and resetting data. The originalstate of the phase change layer 120 is amorphous state with highresistance and presents data “0”. The amorphous state is usuallyreferred to as a reset state. The crystalline state with low resistancepresents data “1”.

During the process of writing data, a voltage is applied between thefirst electrode 132 and the second electrode 134, the carbon nanotubelayer 110 will produce heat to heat the phase change layer 120. Whilethe temperature of the phase change material (GST) is heated to behigher than the Tc, the phase change material will turn to crystallinestate with low resistance. Thus, the process of writing data iscompleted.

Furthermore, when the voltage is removed, the phase change material issettled in the crystalline state when it is slowly quenched after beingheated within a temperature window that higher than the Tc and low thanthe Tm during a time.

During the process of reading data, a relatively small voltage isapplied between the third electrode 136 and the fourth electrode 138.The current flowing in the phase change layer 120 is so small that itcannot cause the phase transition. By measuring the feedback current,the resistance of the phase change layer 120 can be obtained. The datacan be read out by comparing the obtained resistance with the originalresistance of the phase change layer 120. If the obtained resistance issmaller than the original resistance of the phase change layer 120, adata “1” is obtained. If the obtained resistance is substantially equalto the original resistance, a data “0” is obtained.

During the process of resetting data, the phase change layer 120 israpidly quenched after being heated over its melting point by suppliedcurrent. The state of the phase change material will be transferred intothe amorphous state. The amorphous state is usually referred as a resetstate, storing data “0”. Thus the data of the phase change memory cell10 is reset. The resistance in the memory cell is relatively high in theamorphous state, and relatively low in the crystalline state.

Referring to FIG. 3, a method for making a phase change memory cell 10includes following steps:

(S11) providing a substrate 100;

(S12) stacking a carbon nanotube layer 110 and a phase change layer 120on a surface of the substrate 100;

(S13) applying a first electrode 132 and a second electrode 134 in aninterval on the carbon nanotube layer 110;

(S14) locating a third electrode 136 and a fourth electrode 138 in aninterval on the phase change layer 120.

In step (S12), the carbon nanotube layer 110 includes a plurality ofcarbon nanotubes extending substantially parallel with the surface ofthe carbon nanotube layer 110. The extending direction of the carbonnanotubes is substantially parallel with the surface of the substrate100. The carbon nanotube layer 110 is a free-standing structure, and itcan be directly disposed on the substrate 100. The phase change layer120 is located on the surface of the carbon nanotube layer 110 whichaway from the substrate 100. The phase change layer 120 can be disposedvia a method such as screen printing, ion beam deposition, electron beamdeposition, chemical vapor deposition or sputtering. In one embodiment,the phase change layer 120 is formed on the substrate 100 viasputtering.

In step (S13), the first electrode 132, the second electrode 134, thethird electrode 136 and the fourth electrode 138 can be formed by amethod such as screen printing, ion beam deposition, electron beamdeposition, chemical vapor deposition or coating. In one embodiment, thefirst electrode 132, the second electrode 134, the third electrode 136and the fourth electrode 138 are formed by screening printingrespectively. The first electrode 132 and the second electrode 134 arelocated on the two opposite sides of the carbon nanotube layer 110 in acertain interval. The third electrode 136 and the fourth electrode 138are located on the phase change layer 120 in a certain interval. Thephase change layer 120 is located between the carbon nanotube layer 110and both of the third electrode 136 and the fourth electrode 138.

In the method for making the phase change memory cell 10, because thecarbon nanotube layer 110 is a free-standing structure, the carbonnanotube layer 110 can be directly located on the substrate 100 andfunction as the heating element. The manufacturing process is simple.The carbon nanotube layer 110 is stacked with the phase change layer120, thus the mechanical strength will be enhanced and the heatingefficiency will be improved.

Referring to FIG. 4 and FIG. 5, a phase change memory cell 20 includes asubstrate 100, a carbon nanotube layer 110, a phase change layer 120, afirst electrode 132, a second electrode 134, a first row electrode lead142, a second row electrode lead 144, a first column electrode lead 146and a second column electrode lead 148. The phase change memory cell 20includes a first circuit and a second circuit. The first circuitincludes the first row electrode lead 142, the first electrode 132, thecarbon nanotube layer 110 and the first column electrode lead 146electrically connected in series. The first circuit is a data writingcircuit used to write data in the working process of the phase changememory cell 20. The second circuit includes the second row electrodelead 144, the carbon nanotube layer 110, the phase change layer 120 andthe second column electrode lead 148 electrically connected in series.The carbon nanotube layer 110 is at least partly overlapped with thephase change layer 120. The second circuit is a data writing circuitused to read data. Both the first circuit and the second circuit can beused to reset the phase change memory cell 20.

The first row electrode lead 142 and the second row electrode lead 144are parallel with each other and located on the substrate 100 in aninterval. The first column electrode lead 146 and the second columnelectrode lead 148 are parallel with each other and located on thesubstrate 100 in a certain interval. The first row electrode lead 142intersects with and is insulated from the first column electrode lead146 and the second column electrode lead 148. The second row electrodelead 144 intersects with and is insulated from the first columnelectrode lead 146 and the second column electrode lead 148. The phasechange layer 120 and the carbon nanotube layer 110 are stacked at theintersection of the second row electrode lead 144 and the second columnelectrode lead 148. The first row electrode lead 142, the firstelectrode 132, the carbon nanotube layer 110 and the first columnelectrode lead 146 are electrically connected in series to form thefirst circuit. The first circuit is a heating circuit used to heat thephase change layer 120 to write data therein. Thus the first circuit isthe write circuit. The second row electrode lead 144, the carbonnanotube layer 110, the phase change layer 120 and the second columnelectrode lead 148 are electrically connected in series to form thesecond circuit. The second circuit is used to detect the resistance ofthe phase change layer 120 to get the data. Thus the second circuit isthe read circuit.

When applying a voltage between the first row electrode lead 142 and thefirst column electrode lead 146, the current flows through the first rowelectrode lead 142, the carbon nanotube layer 120 and the first columnelectrode lead 146. The data can be written into the phase change memorycell 20. When applying a voltage between the second row electrode lead144 and the second column electrode lead 148, the current flows thoughthe second row electrode lead 144, the carbon nanotube layer 110 and thephase change layer 120 and the second column electrode lead 148. Thedata can be read out from the phase change memory cell 20.

The distance between the first row electrode lead 142 and the second rowelectrode lead 144 can range from about 50 nm to about 2 centimeters.The width of the first row electrode lead 142 and the second rowelectrode lead 144 can range from about 30 nm to about 100 μmrespectively. The thickness of the first row electrode lead 142 and thesecond row electrode lead 144 can range from about 10 nm to about 100 nmrespectively. The material of the first row electrode lead 142 and thesecond row electrode lead 144 can be made of conductive material such asmetal and ITO. In one embodiment, the material of the first rowelectrode lead 142 and the second row electrode lead 144 is made ofconductive pastes and made by screen printing.

The first column electrode lead 146 and the second column electrode lead148 are parallel with each other. The first column electrode lead 146intersects with and is insulated from the first row electrode lead 142by an insulating layer 101. The insulating layer 101 is located betweenthe first column electrode lead 146 and the first row electrode lead 142in the intersection of them. The material of the insulating layer 101can be SiO₂, Si₃N₄ or Ta₂O₅. The thickness of the insulating layer 101ranges from about 50 nm to about 200 μm and can be selected according toneed. An angle between the extending direction of the first columnelectrode lead 146 with the first row electrode lead 142 and the secondrow electrode lead 144 ranges from about 10 degrees to about 90 degrees.In one embodiment, the first column electrode lead 146 is perpendicularwith the first row electrode lead 142 and the second row electrode lead144.

The second column electrode lead 148 intersects and is perpendicularwith the first row electrode lead 142 and the second row electrode lead144. The insulating layer 101 is located between the second columnelectrode lead 148 and the first row electrode lead 142 and the secondrow electrode lead 144. The carbon nanotube layer 110 and the phasechange layer 120 are stacked on the intersection of the second columnelectrode lead 148 and the second row electrode lead 144.

The carbon nanotube layer 110 is located on the intersection of thesecond row electrode lead 144 and the second column electrode lead 148and partly covers and electrically connects with the second rowelectrode lead 144. Furthermore, an insulating layer (not shown) may belocated between the carbon nanotube layer 110 and the second rowelectrode lead 144. The insulating layer is used to reduce the contactsurface between the carbon nanotube layer 110 and the second rowelectrode lead 144. Therefore most of the current will flow through thecarbon nanotubes. The shape of the carbon nanotube layer 110 can betriangle, square, rectangular, round, oval or other shape. The size ofthe carbon nanotube layer 110 is arbitrary. In one embodiment, thecarbon nanotube layer 110 is square. The length of the carbon nanotubelayer 110 ranges from about 50 nm to about 900 nm. The width of thecarbon nanotube layer 110 ranges from about 20 nm to about 600 nm. Inone embodiment, the length is about 70 μm and the width is about 80 μm.The thickness of the carbon nanotube layer 110 ranges from about 0.5 nmto about 100 nm such as 20 nm, 5 μm and 10 μm. In one embodiment, thecarbon nanotube layer 110 is directly applied on the substrate 100. Thecarbon nanotube layer 110 can be etched via lithography, etching, laseror plasma etching electronic to partly reserve the carbon nanotube layerwhich is necessary. Thus the carbon nanotube layer 110 will form apattern on the substrate 100.

The phase change layer 120 is stacked with the carbon nanotube layer110. In one embodiment, the entire surface of the phase change layer 120is in contact with the surface of the carbon nanotube layer 110.Furthermore, a heat conductive layer (not shown) can be located betweenthe phase change layer 120 and the carbon nanotube layer 110. The heatconductive layer is used to conduct the heat from the carbon nanotubelayer 110 to the phase change layer 120.

Furthermore, the phase change layer 120 and the carbon nanotube layer110 are surrounded and covered by a thermal insulation material (notshown). The thermal insulation material can be located on the phasechange layer 120 and the carbon nanotube layer 110 via coating or screenprinting. The thermal insulation material covers part surface of thecarbon nanotube layer 110 and phase change layer 120 which is exposed onthe substrate 100. The thermal insulation material is used to reduce theheat loss, the time to reach the phase transition temperature of thephase change layer 120 will be reduced. Thus the response speed of thephase change memory cell 20 will be improved.

The first electrode 132 is a bar-shape electrode and extends from thefirst row electrode lead 142 to the second row electrode lead 144. Theextending direction of the first electrode 132 is perpendicular withthat of the first row electrode lead 142. The first electrode 132includes two opposite ends. The first end is electrically connected withthe first row electrode lead 142, and the second end is electricallyconnected with the carbon nanotube layer 110. The first electrode 132 isspaced from the second column electrode lead 148. Furthermore, the firstelectrode 132 is insulated from the second row electrode lead 144 withthe insulating layer 101. It is can be understood that the firstelectrode 132 can be omitted, and the first row electrode lead 142 canbe directly connected with the carbon nanotube layer 110.

The second electrode 134 is a bar-shape electrode and extending from thefirst column electrode lead 146 to the second column electrode lead 148.The extending direction of the second electrode 134 is perpendicularwith the first column electrode lead 146. The second electrode 134includes two opposite ends. The first end is electrically connected withthe first column electrode lead 146, and the second end is connected tothe carbon nanotube layer 110. The second electrode 134 is not directlyin contact with the first electrode 132, thus the first electrode 132,the carbon nanotube layer 110 and the second electrode 134 areelectrically connected in series to form a working circuit. The firstcolumn electrode lead 146, the carbon nanotube layer 110 and the firstrow electrode lead 142 are electrically connected in series via thisworking circuit to form the first circuit. The first circuit is used towrite data in the phase change memory cell 20. It is can be understoodthat the second electrode 134 is a selective element, and the firstcolumn electrode lead 146 can be directly connected with the carbonnanotube layer 110.

During the working process of the phase change memory cell 20, The firstrow electrode lead 142 is functioned as the first writing electrode, thesecond row electrode lead 144 is used as the first reading electrode,the first column electrode lead 146 is used as the second writingelectrode and the second column electrode lead 148 is used as the secondreading electrode.

When writing data into the phase change memory cell 20, a voltage or asignal is applied between the first writing electrode and the secondwriting electrode. Thus a current is introduced into the first circuit.The carbon nanotube layer 110 begins to produce heat to heat the phasechange layer 120 in original phase state.

When reading data from the phase change memory cell 20, a voltage isapplied between the first reading electrode and the second readingelectrode, a current is introduced into the second circuit. The currentis a relatively small therefore cannot cause the phase transition. Bymeasuring the current flowing in the second circuit, the resistance ofthe phase change layer 120 can be calculated out. The phase state of thephase change layer 120 can be obtained by compared this resistance withthat of the original state of the phase change layer 120. Thus the datais obtained.

Referring to FIG. 6, a method for making the phase change memory cell 20includes following steps:

(S21) providing a substrate 100;

(S22) applying a first row electrode lead 142 and a second row electrodelead 144 on a surface of the substrate 100;

(S23) locating a carbon nanotube layer 110 on the substrate, wherein thecarbon nanotube layer 110 contacts the second row electrode lead 144;

(S24) attaching a phase change layer 120 on a surface of the carbonnanotube layer 110;

(S25) locating a first column electrode lead 146, a second columnelectrode lead 148, a first electrode 132 and a second electrode 134 onthe substrate 100, wherein the first electrode 132 is electricallyconnected with the first row electrode lead 142 and the carbon nanotubelayer 110, the second electrode 134 is electrically connected with thefirst column electrode lead 146 and the carbon nanotube layer 110, andthe second column electrode lead 148 is in contact with the phase changelayer 120.

In step (S21), the substrate 100 is an insulated substrate. In oneembodiment, the substrate 100 is a flexible substrate.

In step (S22), the first row electrode lead 142 and the second rowelectrode lead 144 can be fabricated via screen printing, ion beamdeposition, electron beam deposition and coating. The first rowelectrode lead 142 and the second row electrode lead 144 are parallelwith and spaced from each other.

In step (S23), the carbon nanotube layer 110 is a continuous andfree-standing structure. The carbon nanotube layer 110 can be formed bydirectly applying a carbon nanotube film on the substrate 100. Thecarbon nanotube film is drawn from a carbon nanotube array. The carbonnanotube layer 110 partly covers the second row electrode lead 144 andelectrically connects with it.

In step (S25), the method of making the first column electrode lead 146,the second column electrode lead 148, the first electrode 132 and thesecond electrode 134 is same as that of the first row electrode lead142. The first column electrode lead 146 is spaced from the first rowelectrode lead 142 and the second row electrode lead 144 via aninsulating layer 101. The insulating layer 101 is applied on theintersection between the first column electrode lead 146 and the firstrow electrode lead 142. The insulating layer 101 is also located betweenthe first column electrode lead 146 and the second row electrode lead144.

Referring to FIG. 7 and FIG. 8, a phase change memory cell 30 includes asubstrate 100, a carbon nanotube layer 110, a phase change layer 120, afirst electrode 132, a second electrode 134, a first row electrode lead142, a second row electrode lead 144, a first column electrode lead 146and a second column electrode lead 148. The structure of the phasechange memory cell 30 is similar to that of the phase change memory cell20. The difference is that in the phase change memory cell 30, the phasechange layer 120 is located on the substrate 100 and in contact with thesecond row electrode lead 144, and the carbon nanotube layer 110 islocated on the phase change layer 120 and electrically connected withthe second column electrode lead 148.

The phase change layer 120 is located on the substrate 100 and partlycovers the second row electrode lead 144. The carbon nanotube layer 110is located on the intersection of the second row electrode lead 144 andthe second column electrode lead 148 and located between them.Furthermore, the carbon nanotube layer 110 is electrically connectedwith the second column electrode lead 148. The first row electrode lead142, the first electrode 132, the carbon nanotube layer 110, the secondelectrode 134 and the first column electrode lead 146 are electricallyconnected in series to form the first circuit. The second columnelectrode lead 148, the carbon nanotube layer 110, the phase changelayer 120, and the second row electrode lead 144 are electricallyconnected in series to form the second circuit. The area of the carbonnanotube layer 110 is greater than that of the phase change layer 120,and the entire surface of the phase change layer 120 is absolutelycovered by the carbon nanotube layer 110.

A method for making a phase change memory cell 30 includes followingsteps:

(S31) providing a substrate 100;

(S32) locating a first row electrode lead 142 and a second row electrodelead 144 on a surface of the substrate 100;

(S33) applying a phase change layer 120 and connecting the phase changelayer 120 with the second row electrode lead 144;

(S34) attaching a carbon nanotube layer 110 on the surface of the phasechange layer 120;

(S35) forming a first column electrode lead 146, a second columnelectrode lead 148, a first electrode 132 and a second electrode 134 onthe substrate 100, wherein the first electrode 132 is electricallyconnected with the first row electrode lead 142 and the carbon nanotubelayer 110, the second electrode 134 is electrically connected with thefirst column electrode lead 146 and the carbon nanotube layer 110, andthe second column electrode lead 148 is in contact with the carbonnanotube layer 110.

The method of making the phase change memory cell 30 is similar to thatof the phase change memory cell 20, except that the phase change layer120 is firstly applied on the substrate and the carbon nanotube layer110 covers the phase change layer 120.

Referring to FIG. 9 and FIG. 10, a phase change memory cell 40 includesa substrate 100, a carbon nanotube layer 110, a first phase change layer1201, a second phase change layer 1202, a first electrode 132, a secondelectrode 134, a first row electrode lead 142, a second row electrodelead 144, a first column electrode lead 146 and a second columnelectrode lead 148. The structure of the phase change memory cell 40 issimilar to that of the phase change memory cell 30. The difference isthat in the phase change memory cell 40, a second phase change layer1202 is located on the carbon nanotube layer 110 and electricallyconnected with the second column electrode lead 148, the carbon nanotubelayer 110 is located between the first phase change layer 1201 and thesecond phase change layer 1202.

The first row electrode lead 142, the first electrode 132, the carbonnanotube layer 110, the second electrode 134 and the first columnelectrode lead 146 are electrically connected in series to form thefirst circuit. The second column electrode lead 148, the first phasechange layer 1201, the carbon nanotube layer 110, the second phasechange layer 1202, and the second row electrode lead 144 areelectrically connected in series to form the second circuit.

While the phase change memory cell 40 is working, if one of the phasechange layers cannot normally change the phase state, another phasechange layer can still work normally. Thus the phase change memory cell40 can still keep working Thus the life span of the phase change memorycell 40 will be prolonged and the reliability will be improved.

A method for making a phase change memory cell 30 includes followingsteps:

(S41) providing a substrate 100;

(S42) locating a first row electrode lead 142 and a second row electrodelead 144 on a surface of the substrate;

(S43) applying a first phase change layer 1201 and connecting the firstphase change layer 1201 with the second row electrode lead 144;

(S44) locating a carbon nanotube layer 110 on the surface of the firstphase change layer 1201;

(S45) locating a second phase change layer 1202 on the carbon nanotubelayer 110;

(S46) locating a first column electrode lead 146, a second columnelectrode lead 148, a first electrode 132 and a second electrode 134 onthe substrate 100, wherein the first electrode 132 is electricallyconnected with the first row electrode lead 142 and the carbon nanotubelayer 110, the second electrode 134 is electrically connected with thefirst column electrode lead 146 and the carbon nanotube layer 110, andthe second column electrode lead 148 is in contact with the second phasechange layer 1202.

The method of making the phase change memory cell 40 is similar to thatof making the phase change memory cell 30. The difference is that a stepof applying another phase change layer on the carbon nanotube layerbefore the step (S46).

Referring to FIG. 11, a phase change memory 50 includes a substrate 100,a plurality of first row electrode leads 142, a plurality of second rowelectrode leads 144, a plurality of first column electrode leads 146, aplurality of second column electrode leads 148 and a plurality of phasechange memory units 104 located on the substrate 100.

The plurality of first row electrode leads 142 is parallel and spacedwith each other. The plurality of second row electrode leads 144 isparallel and spaced with each other. The first row electrode leads 142and the second row electrode leads 144 are located alternately in thedirection which is perpendicular to the extending direction. It meansthat, one first row electrode lead 142 is located between two adjacentsecond row electrode leads 144, and one second row electrode lead 144 islocated between two adjacent first row electrode leads 142. Similarly,the first column electrode lead 146 and the second column electrode lead148 are located alternately in the direction which is perpendicular tothe extending direction. The first column electrode lead 146 isintersected and insulated with each of the first row electrode lead 142and the second row electrode lead 144. The second column electrode lead148 is also intersected and insulated with each of the first rowelectrode lead 142 and the second row electrode lead 144. A section 102is defined between the adjacent first row electrode lead 142, the secondrow electrode lead 144, the first column electrode lead 146 and thesecond column electrode lead 148. The phase change memory unit 104 isreceived in one of sections 102.

Each phase change memory unit 104 includes a first electrode 132, asecond electrode 134, a carbon nanotube layer 110 and a phase changelayer 120.

Each section 102 and the phase change memory unit 104 forms a phasechange memory cell. The structure of the phase change memory cell can besame as the phase change memory cell 20, 30, 40 mentioned above. In oneembodiment, the plurality of the phase change memory units 104 isaligned in an array. The phase change memory units 104 in the same roware electrically connected with the same first row electrode lead 142and the same second row electrode lead 144. The phase change memoryunits 104 in the same row are electrically connected with the same firstcolumn electrode lead 146 and the same second column electrode lead 144.Each of The phase change memory units 104 can be separately controlled.The phase change memory units 104 can be aligned in a relatively highdensity, thus it can be used for large-capacity data storage andreading.

Referring to FIG. 12, a method for making the phase change memory 50includes following steps:

(S51) providing a substrate 100;

(S52) alternatively locating a plurality of first row electrode lead 142and second row electrode lead 144 on a surface of the substrate 100;

(S53) applying a carbon nanotube layer 110 on the substrate 100 to coverthe first row electrode lead 142 and second row electrode lead 144;

(S54) patterning the carbon nanotube layer 110 to form a plurality ofcarbon nanotube units 1101;

(S55) attaching a phase change layer 120 on each of the carbon nanotubeunits 1101;

(S56) forming a first column electrode lead 146, a second columnelectrode lead 148, a first electrode 132 and a second electrode 134 onthe substrate 100, the first electrode 132 is electrically connectedwith the first row electrode lead 142 and the carbon nanotube unit 1101,the second electrode 134 is electrically connected with the first columnelectrode lead 146 and the carbon nanotube unit 1101, and the secondcolumn electrode lead 148 is in contact with the carbon nanotube unit1101.

In step (S52), the plurality of first row electrode leads 142 and thesecond row electrode leads 144 can be formed by following method: first,applying a plurality of electrode leads parallel with each other on thesubstrate 100; second, alternately defining the first row electrode lead142 and the second row electrode lead 144 in the plurality electrodeleads. It means that one electrode leads is defined as the first rowelectrode lead 142, and another adjacent electrode leads will be definedas the second row electrode lead 144 and so on.

In step (S53), the carbon nanotube layer 110 covers the entire surfaceof the substrate 100, and covers the first row electrode lead 142 andthe second row electrode lead 144.

In step (S54), the carbon nanotube layer 110 can be etched via a methodof lithography, etching or reactive ion etching (RIE). The carbonnanotube layer 110 can be partly reserved according to need. Thus thecarbon nanotube layer 110 is divided into a plurality of carbon nanotubeunits 1101. The carbon nanotube units 1101 are spaced from each otherand partly cover the surface of the second row electrode lead 144.

In one embodiment, the step of patterning the carbon nanotube layer 110includes following substeps:

(S541) providing a laser device; and

(S542) irradiating the carbon nanotube layer 110 with the laser toremove part of the carbon nanotube layer 110 to form a plurality of thecarbon nanotube units 1101.

In step (S541), the laser device can be an argon ion laser or a carbondioxide laser. The laser device can emit a pulse laser beam. The powerof the laser ranges from about 1 watt to about 100 watts. The powerdensity of the laser irradiating on the carbon nanotube layer 110 isabout 0.053×10¹² watt/m². In one embodiment, the power of the laser isabout 12 watts. The laser beam is irradiated on the carbon nanotubelayer 120, and a laser spot can be formed on the carbon nanotube layer120. The shape of the laser spot is substantially round with a diameterranging from about 1 μm to about 5 millimeters.

In step (S542), while the carbon nanotube layer 110 is irradiated by thelaser, a plurality of carbon nanotubes on the irradiated position willbe burn down, and other carbon nanotubes will be reserved. Then aplurality of carbon nanotube layer unit 1101 will be formed into apatterned carbon nanotube layer 110. During the process of irradiatingthe carbon nanotube layer 110, the energy of the laser is focus on thecarbon nanotube layer 110. Other elements on the substrate are notaffected by the laser. The incident angle of the laser is arbitrary. Inone embodiment, the incident direction is perpendicular with the carbonnanotube layer 110.

Furthermore, the carbon nanotube layer 110 can also patterned via RIE toform a plurality of carbon nanotube layer unit 1101. The carbon nanotubelayer 110 is etched by following steps:

providing a mask layer (not shown);

locating the mask layer on the carbon nanotube layer 110; and

etching the carbon nanotube layer via RIE to form a plurality of carbonnanotube unit 1101.

The etching gas can be selected according to need. The etching gas cancorrupt the carbon nanotube layer 110. The mask layer should not beaffected by the etching gas. In one embodiment, the etching gas is CF₄and SF₆. The flow rate of the CF₄ ranges from about 10 sccm to about 50sccm. The flow rate of the SF₆ ranges from about 2 sccm to about 20sccm. During the etching process, the unwanted carbon nanotubes will beremoved to form the plurality of the carbon nanotube unit 1101.

In the method of making the phase change memory 50, the carbon nanotubelayer 110 is a free-standing structure, thus it can be directly disposedon the substrate. The carbon nanotube layer can be easily etched to forma plurality of phase change memory cell, thus it is convenient tofabricate the phase change memory with a relatively large area.Furthermore, the carbon nanotube layer is etched via the lithography orRIE to form the carbon nanotube unit, the precision and the size of thecarbon nanotube unit can be precisely controlled. Thus a phase changememory cell with relatively small size can be formed, and the densityand the integration of the phase change memory cell will be improved.

The phase change memory cell and the phase change memory can havefollowing advantages. First, the carbon nanotube layer has relativelyhigh conductivity and chemical stability, thus the life span of thephase change memory will be improved. Second, the carbon nanotube layeris functioned as the heating element, and the carbon nanotube layer isflexible, thus the phase change memory can be flexible too. Third, thecarbon nanotube film is a free-standing structure with suitableconductivity, thus the current can be introduced into the carbonnanotube layer via applying a voltage on any two opposite sides of thecarbon nanotube layer, and it is convenient to set the electrode.Fourth, because the carbon nanotube layer can be directly applied on thesubstrate and easily etched via lithography to form the heating element,the method is relatively simple, the integration is relatively high andthe cost is relatively low.

Depending on the embodiment, certain of the steps of methods describedmay be removed, others may be added, and the sequence of steps may bealtered. It is also to be understood that the description and the claimsdrawn to a method may include some indication in reference to certainsteps. However, the indication used is only to be viewed foridentification purposes and not as a suggestion as to an order for thesteps.

It is to be understood that the above-described embodiments are intendedto illustrate rather than limit the disclosure. Variations may be madeto the embodiments without departing from the spirit of the disclosureas claimed. It is understood that any element of any one embodiment isconsidered to be disclosed to be incorporated with any other embodiment.The above-described embodiments illustrate the scope of the disclosurebut do not restrict the scope of the disclosure.

What is claimed is:
 1. A phase change memory cell, comprising: a firstcarbon nanotube layer; a phase change layer, wherein at least part ofthe phase change layer is overlapped with the first carbon nanotubelayer; a first electrode and a second electrode electrically connectedwith the first carbon nanotube layer, wherein the first electrode andthe second electrode are configured to apply a first voltage to thefirst carbon nanotube layer; and a third electrode electricallyconnected with the phase change layer, wherein the third electrode andthe first electrode are configured to apply a second voltage to thephase change layer.
 2. The phase change memory cell of claim 1, whereinthe first carbon nanotube layer is a free-standing structure.
 3. Thephase change memory cell of claim 1, wherein the first carbon nanotubelayer is a continuous layer-shaped structure and comprises a pluralityof carbon nanotubes joined end to end by van der Waals force, and anextending direction of each of the plurality of carbon nanotubes isparallel with a surface of the first carbon nanotube layer.
 4. The phasechange memory cell of claim 3, further comprising a substrate, whereinthe phase change layer and the first carbon nanotube layer are stackedon a surface of the substrate.
 5. The phase change memory cell of claim4, wherein the substrate is flexible.
 6. The phase change memory cell ofclaim 4, wherein the extending direction of each of the plurality ofcarbon nanotubes is parallel with the surface of the substrate.
 7. Thephase change memory cell of claim 1, further comprising another phasechange layer on the first carbon nanotube layer, and the first carbonnanotube layer is located between the two phase change layers.
 8. Thephase change memory cell of claim 1, wherein the phase change layer islocated within a coverage area of the first carbon nanotube layer. 9.The phase change memory cell of claim 1, further comprising a secondcarbon nanotube layer on the phase change layer, and the phase changelayer is located between the first carbon nanotube layer and the secondcarbon nanotube layer.
 10. The phase change memory cell of claim 1,wherein the first carbon nanotube layer comprises a drawn carbonnanotube film, the drawn carbon nanotube film comprises a plurality ofcarbon nanotubes are joined end to end by van der Waals force and areoriented along the same direction.
 11. The phase change memory cell ofclaim 10, wherein the first carbon nanotube layer comprises a pluralityof drawn carbon nanotube films stacked together, and extendingdirections of the plurality of carbon nanotubes in adjacent drawn carbonnanotube films are substantially the same.
 12. The phase change memorycell of claim 10, wherein the first carbon nanotube layer comprises aplurality of drawn carbon nanotube films stacked together and extendingdirections of the plurality of carbon nanotubes in adjacent drawn carbonnanotube films intersected with each other.
 13. The phase change memorycell of claim 1, wherein the first carbon nanotube layer comprises aflocculated carbon nanotube film, and the flocculated carbon nanotubefilm comprises a plurality of carbon nanotubes entangled with eachother.
 14. The phase change memory cell of claim 1, wherein a thicknessof the first carbon nanotube layer ranges from about 0.5 nm to about 100μm.
 15. The phase change memory cell of claim 1, wherein a thickness ofthe phase change layer ranges from about 10 nm to about 150 nm.
 16. Thephase change memory cell of claim 1, wherein the third electrode, thephase change layer, the first carbon nanotube layer and the firstelectrode are electrically connected in series to form a circuitconfigured to read data from the phase change memory cell.
 17. A phasechange memory cell, comprising: a first electrode; a second electrode,wherein the second electrode is intersected with and spaced from thefirst electrode to define an intersection; a first phase change layerand a carbon nanotube layer stacked together and sandwiched between thefirst electrode and the second electrode at the intersection, whereinthe first electrode and the second electrode are configured to apply afirst voltage to the phase change layer; a third electrode and a fourthelectrode are electrically connected with the carbon nanotube layer,wherein the third electrode and the fourth electrode are configured toapply a second voltage to the carbon nanotube layer.
 18. The phasechange memory cell of claim 17, wherein the carbon nanotube layer andthe phase change layer are electrically connected in series between thefirst electrode and the second electrode.
 19. A phase change memory,comprising: a substrate; a plurality of row electrode leadssubstantially parallel to each other and located on the substrate; aplurality of column electrode leads substantially parallel to each otherand located on the substrate, wherein the plurality of column electrodeleads are set an angle relative to the plurality row electrode leads todefine a grid having a plurality of sections; a plurality of phasechange memory cells, wherein each phase change memory cell is receivedin one of the plurality of sections, the phase change memory cellcomprises a carbon nanotube layer, a phase change layer, a firstelectrode and a second electrode electrically connected with the carbonnanotube layer, and at least part of the phase change layer isoverlapped with the carbon nanotube layer; the first electrode and thesecond electrode are adapted to apply a first voltage to the carbonnanotube layer; and one of the plurality of column electrode leads iselectrically connected with the phase change layer, wherein one of theplurality of column electrode leads and the first electrode areconfigured to apply a second voltage to the phase change layer.
 20. Thephase change memory of claim 19, wherein the phase change memory cellsin the same row are electrically connected to the same row electrodelead via the first electrode, and the phase change memory cells in thesame column are electrically connected to the same column electrode leadvia the second electrode.